Hydrogen is utilized extensively in industrial processes. For example, large quantities of hydrogen are utilized in the synthesis of ammonia, methanol, and the like; for hydrorefining and hydrotreating petroleum; for catalytic hydrogenation; for food hydrogenation; for metal annealing; for the formation of hydrogen peroxide; and in fuel cells for the generation of electricity. Carbon dioxide is also utilized significantly in industrial processes. For example, carbon dioxide may be injected into oil wells to enhance oil recovery; may be used to enhance coal bed treatments; may be used in molten carbonate fuel cells; and may be utilized in greenhouses.
There are numerous processes for producing hydrogen and carbon dioxide from hydrocarbons or carbonaceous materials. A first step may involve producing hydrogen and carbon monoxide. For example, a gaseous stream containing hydrogen and carbon monoxide may be produced by steam reforming hydrocarbon materials such as methane; reacting coke or coal with steam and air; or partially oxidizing hydrocarbons such as methane, kerosene or diesel to produce a gas stream containing hydrogen and carbon monoxide. In a second step, the gas streams containing hydrogen and carbon monoxide may be treated in a water gas shift reaction with steam to produce carbon dioxide and additional hydrogen.
One such process for producing hydrogen and carbon dioxide is disclosed in U.S. Patent Publication No. 2003/0068269. A feed stream of methane is both steam-reformed and water-gas shifted in a single reactor in the presence of a hydrogen-selective, hydrogen-permeable membrane. Hydrogen gas is selectively separated from the product stream by diffusion through the membrane, driving the equilibrium of both of the reactions towards product gases. The temperature of the reactor may be controlled to favor production of carbon dioxide together with hydrogen. For example, the reactor may be run at 500° C. to favor producing shift reaction products carbon dioxide and hydrogen, as opposed to producing steam reforming products carbon monoxide and hydrogen which occurs at higher temperatures. The reactor cannot be run efficiently at temperatures much lower than 500° C. since some steam reforming must occur to convert the hydrocarbon to hydrogen and carbon monoxide, and the reforming reaction is a high temperature reaction, typically run at 700 to 1100° C. The high reaction temperature conditions necessary to conduct the steam reforming reaction, however, are detrimental to the lifespan of the hydrogen-selective, hydrogen-permeable membrane since such temperatures are near the operational limit of such membranes. Further, the process may not provide highly efficient energy conversion of the hydrocarbons to hydrogen and carbon dioxide since the process may not convert most or substantially all of the hydrocarbons to hydrogen and carbon dioxide since the reactor must be run at temperatures that only inefficiently steam reform hydrocarbons in order to simultaneously conduct a shift reaction.
A process for producing a mixture of hydrogen and carbon dioxide from a hydrocarbon or carbonaceous feedstock is provided in U.S. Pat. No. 6,090,312. A hydrocarbon feed, including a feed derived by reacting carbon dioxide with steam, is steam-reformed and shift reacted in a reactor to produce a gas stream containing hydrogen and carbon dioxide along with carbon monoxide, steam and methane. The gas stream is cooled in a heat exchanger sufficiently to condense and remove the steam from the gas stream. Hydrogen and carbon dioxide are separated from the gas stream together by passing the gas stream over a membrane selectively permeable to carbon dioxide and hydrogen. The remainder of the gas stream (e.g., methane and carbon monoxide) is reheated and either recycled back into the reactor or passed to a second reactor for further steam-reformation and shift reaction to produce more hydrogen and carbon dioxide. The subsequently produced hydrogen and carbon dioxide are separated from the resultant gas stream together by a membrane selectively permeable to carbon dioxide and hydrogen. Although the residual hydrocarbon stream and carbon monoxide are recycled or further processed in the process, the process is inefficient for energy conversion of hydrocarbons to hydrogen and carbon dioxide since the competing steam reforming and shift reactions are conducted in the same reactor with no means within the reactor for driving the equilibrium of either reaction towards the production of hydrogen and carbon dioxide.
A process for separating hydrogen and carbon dioxide from a feed stream containing hydrogen and carbon monoxide is disclosed in U.S. Pat. No. 3,251,652. Gaseous mixtures containing hydrogen and carbon monoxide derived from hydrocarbon or carbonaceous feedstocks are contacted with a hydrogen-permeable diffusion membrane for the separation of a pure diffused hydrogen stream. The undiffused gases are then processed in a water-gas shift reactor to convert carbon monoxide to carbon dioxide and produce more hydrogen. The resultant gas product contains carbon dioxide, hydrogen, and, at the least, some carbon monoxide. The hydrogen may be separated from the other gases by contact with a second hydrogen-permeable diffusion membrane, or it may be recycled back into the shift reactor after removal of carbon dioxide by refrigeration. The process does not energy efficiently convert most or all hydrocarbons in a feed to hydrogen and carbon dioxide since any hydrocarbon that was not converted into hydrogen and carbon monoxide in producing the hydrogen and carbon monoxide feedstock remains unconverted throughout the process since the hydrocarbon is not reformed into hydrogen and carbon monoxide by the shift reactor.
It would be desirable to have a system and a process for efficiently converting most or substantially all of a hydrocarbon feedstock to hydrogen and carbon dioxide, thereby providing improved energy conversion of the hydrocarbon feedstock into hydrogen and carbon dioxide. It would also be desirable to provide a process and a system capable of achieving such efficient conversion where the process and system provide for separating hydrogen from a gas stream at a temperature below the temperatures required to steam reform a hydrocarbon.